It seems to be true that "sexual restrictions" have been imposed upon Japanese society mostly by external influences: during Meiji Era (in Japan's bid for westernization) as well as in the postwar period: prostitution was outlawed in 1958, so most of the famous red-light districts had to close down, also in the light of the Olympics that were to be held in Tokyo in 1964. Well, "officially" outlawed...

I wouldn't restrict sexual naivity or negligence to girls only. Just think of the hordes of salarymen invading SE Asian countries in search of sexual pleasures. A large amount of Japanese HIV-positive contracted the disease during such holidays.